1. Field of the Invention
The invention relates to an apparatus for treating a material web, which has at least one nip through which the material web is guided and which is formed between a first revolving, heated surface and a second revolving surface, it being possible for the first surface to be heated to at least 120° C. and both surfaces in the nip having the same direction of movement as the material web. Furthermore, the invention relates to a method for treating a material web, in which the material web is guided through a nip which is formed between a first revolving, heated surface and a second revolving surface, the first surface being heated to at least 120° C.
In the following text, the invention will be described by using the example of a paper web. However, it can also be used with other material webs in which similar problems occur.
2. Discussion of Background Information
In order to improve surface quality and for the purpose of consolidation, paper webs are guided through at least one nip which, as a rule, is formed in what is known as a calender. In this case, the nip can be formed by two rolls, of which one is heated. The other roll then often has a resilient surface. However, the nip can also be formed by a heated roll and a shoe roll interacting therewith. The shoe roll has a cover which is brought into contact with the opposing roll over a predetermined circumferential section by a pressure element. Other configurations of the nip are of course likewise conceivable, for example a configuration in which the nip is formed by two circulating belts.
It is known that the surface quality can be improved if heat is supplied to the paper web in the nip. For this purpose, one of the two surfaces is heated, for example the surface of a heated roll. Although, in principle, this has positive effects on the surface of the paper web, it can lead to the problems discussed below.
At the end of the nip, the operating pressure is reduced, more or less abruptly, to the ambient pressure. Then, if the paper still has a temperature above the boiling point of the water contained at approximately ambient pressure, that is to say about 1 bar, the water evaporates abruptly. This is already intrinsically a problem, since the moisture of the paper or board web can thus fall below a desired level. This can, for example, impair downstream processes, such as a printing process. On the other hand, on account of technical restrictions in upstream processes, it is often not possible to increase the web moisture upstream of the nip in order to remedy this problem.
Furthermore, the water evaporating abruptly can lead to fibres being swept along and caused to stand up, and thus to a worsening of the surface quality. This applies even if the fibres are not torn completely out of the paper web.